1. Field of the Invention
The present invention relates generally to a wireless communication system, and in particular, to a method of more efficiently transmitting service data in a Wireless Local Area Network (WLAN) system and a system therefor.
2. Description of the Related Art
Recently, accompanying the development of wireless communication technology and the spread of wireless devices, there have been significant increases in the demands for high speed, reliable data transmission through a wireless link. A WLAN developed in response to the demands generally includes Stations (STAs), which are portable data communication devices, and Access Points (APs) for exchanging data with the STAs. An AP and STAs locating in the same wireless service coverage are called a Base Service Set (BSS).
Each of STAs locating in a single wireless service coverage area transmits or receives data using a wireless resource assigned by an AP. The AP assigns the wireless resource as a type of phase resource indicating a period of time for which an STA or the AP can transmit data.
FIG. 1 illustrates a configuration of a WLAN system to which the present invention is applied.
Referring to FIG. 1, APs 20 and 21 are connected to a wired network 10, and each of a plurality of STAs 32, 34, 36, 42 and 44 transmits or receives data via each of a plurality of wireless channels by being connected to the AP 20 or 21 via a wireless link according to an IEEE 802.11 series Physical (PHY) layer and a Media Access Control (MAC) protocol. The STAs 32, 34 and 36 located in a wireless service coverage 30 and the AP 20 form a single BSS, and the STAs 42 and 44 located in a wireless service coverage 40 and the AP 21 form another single BSS. STAs located in the same wireless service coverage can exchange data via an AP in the same BSS. Major functions of the APs 20 and 21 are transport of data traffic, access to another network (e.g., the wired network 10), support of roaming, synchronization in the same BSS, support of power management and MAC for supporting a time-bound service in the same BSS.
In particular, a MAC layer of each of the APs 20 and 21 or each of the plurality of STAs 32, 34, 36, 42, and 44 controls data transmission and acts as a core element in the WLAN system. The MAC layer defines a Distributed Coordination Function (DCF), which controls every STA requiring a wireless media access in an asynchronous transmission method to have a single First-Input First-Output (FIFO) transmission queue.
Thus, in a DCF mode, the MAC layer examines whether a wireless medium is busy and considers a certain back-off time at an end time of each frame after a certain STA uses a channel, in order to prevent collision of the wireless medium. The MAC layer also determines, using a positive acknowledgement responding to a frame transmitted through the wireless medium, whether a re-transmission request of the transmitted frame is input from a recipient.
The MAC layer uses an Inter-Frame Space (IFS) to define the least amount of time for waiting until a next operation after sensing that the wireless medium is in an idle state. Various types of priorities are provided using the IFS, wherein a smaller IFS value causes a higher priority.
A Distributed IFS (DIFS) indicates a time interval used to transmit user data or a management frame of an STA operating in the DCF mode. A Shorter IFS (SIFS) indicates a highest priority time interval from when a single frame is transmitted to when an Acknowledgement (ACK) frame responding to the frame is transmitted. The SIFS has a fixed value according to a PHY layer considering the time taken until an STA can receive another frame after transmitting a frame.
FIG. 2 is a diagram for explaining a fragmentation method in which a MAC layer transmits data in a WLAN system.
A MAC layer, which has received data from an upper layer, e.g., a Logical Link Control (LLC) layer, divides a MAC Service Data Unit (MSDU) into smaller frame fragments than an original frame and transmits the divided frame fragments. That is, since a long frame has limited reception reliability according to a channel state, transmission reliability of the long frame can be secured by dividing the long frame into small frame fragments using the fragmentation method.
Referring to FIG. 2, the MAC layer compares a received MSDU 100 to a fragmentation threshold, which is a parameter used in the MAC layer, and if the size of the MSDU 100 is greater than the fragmentation threshold, the MAC layer divides the MSDU 100 into small MAC fragments. Herein, the size of the minimum MAC fragment can be set to 256 bytes. Each of the MAC fragments includes a MAC frame (MPDU: MAC Packet Data Unit) 110, 120 or 130 having a MAC header. The MPDUs 110, 120 and 130 are transferred to a PHY layer, and a Physical Layer Convergence Protocol (PLCP) header and a preamble are added to each of the MPDUs 110, 120 and 130 in the PHY layer. A PHY header 140, i.e., the PLCP header and the preamble, and each of the MPDUs 110, 120 and 130 are called a PLCP Protocol Data Unit (PPDU) and are transmitted to a recipient via a wireless channel.
Thus, the MSDU 100 transmitted by the MAC layer in a time axis can be described as follows.
The MAC layer, which has sensed that a wireless medium is in the idle state, transmits a first MPDU 110 including the PHY header 140, i.e., a first PPDU, after considering a back-off time 20 next to a DIFS 50. After waiting for an SIFS 55, the MAC layer receives an ACK 160 from the recipient in response to the first MPDU 110 including the PHY header 140. After waiting for another SIFS 55, the MAC layer transmits a second MPDU 120 including the PHY header 140. After waiting for another SIFS 55, the MAC layer receives an ACK 170 from the recipient in response to the second MPDU 120. After waiting for another SIFS 55, the MAC layer transmits a third MPDU 130 including the PHY header 140. After waiting for another SIFS 55, the MAC layer receives an ACK 180 from the recipient in response to the third MPDU 130.
The MAC layer, which has completely transmitted the MSDU 100, transmits a subsequent MSDU through the above-described transmission procedures after considering another DIFS 50.
As described above, the fragmentation method guarantees the transmission reliability of each of the MPDUs 110, 120, and 130 by dividing the single MSDU 100 into a plurality of small MPDUs 110, 120 and 130, sequentially transmitting the MPDUs 110, 120 and 130, and receiving each of the ACKs 160, 170 and 180 responding to each of the MPDUs 110, 120 and 130 after waiting for the SIFS 55. On the contrary, the fragmentation method delays data transmission due to a time delay corresponding to the length of the SIFS 55 in each of the ACKs 160, 170 and 180 responding to each of the MPDUs 110, 120, and 130, resulting in a decrease of a system performance.
FIG. 3 is a diagram for explaining a Block Acknowledgement (BA) method in which a MAC layer transmits data in a WLAN system. The BA method is a method of transmitting a frame including a plurality of divided small MAC frames and transmitting a subsequent frame without receiving ACKs responding to the MAC frames. Herein, an STA receives a bitmap type BA indicating whether the MAC frames were successfully transmitted.
Referring to FIG. 3, the MAC layer, which has sensed that a wireless medium is in the idle state, sequentially transmits a first MPDU 200 including a PHY header 240, a second MPDU 210 including the PHY header 240, a third MPDU 220 including the PHY header 240 and a fourth MPDU 230 including the PHY header 240 after considering a back-off time 20 next to a DIFS 50. Thereafter, the MAC layer requests a recipient for an ACK signal responding to the transmitted first to fourth MPDUs 200 to 230 in order to confirm whether the recipient has normally received the transmitted first to fourth MPDUs 200 to 230. Herein, the ACK signal responding to the transmitted first to fourth MPDUs 200 to 230 is requested using a single Block ACK Request (BAR) 250. The BAR 250 can include identification information for identifying the first MPDU 200 to the last MPDU 230.
After a time period elapses, the MAC layer receives a BA 260 corresponding to the BAR 250 from the recipient. The BA 260 expresses using a single bitmap, such as ‘1011’, whether the transmitted first to fourth MPDUs 200 to 230 have been successfully received, by assigning a single bit to each of the transmitted first to fourth MPDUs 200 to 230.
The MAC layer confirms using the BA 260 that the second MPDU 210 has not been successfully transmitted and re-transmits the second MPDU 210 including the PHY header 240. Thereafter, the MAC layer requests the recipient for another BAR 250 responding to the second MPDU 210, receives another BA 260 from the recipient, and confirms that the second MPDU 210 has been successfully transmitted.
As described above, a plurality of MPDUs are completely transmitted without receiving an ACK responding to each of the plurality of MPDUs, and a single BA is received in response to the plurality of MPDUs, and thus, a waste of channels required to transmit ACKs can be prevented, thereby increasing transmission efficiency. However, if the BA received from a recipient is delayed, subsequent MPDUs cannot be transmitted for the delay time, also resulting in a transmission delay.
FIG. 4 is a diagram for explaining an aggregation method in which a MAC layer transmits data in a WLAN system. The aggregation method is a method of transmitting a single Physical Service Data Unit (PSDU) by joining a plurality of (e.g., at least 10) MPDUs.
Referring to FIG. 4, the MAC layer transmits a single PSDU obtained by joining at least 10 MPDUs into a single transmission frame to a recipient. Herein, each of the MPDUs joined to form the single transmission frame includes an MPDU delimiter field 300 for distinguishing it from other MPDUs, an MPDU field 310 and a padding field 320.
The MPDU delimiter field 300 disposed in a fore-end of each of the MPDUs has a 4-byte length, and the padding field 320 has a 4-byte length except the last MPDU, i.e., the tenth MPDU. The MPDU delimiter field 300 includes a 4-bit reserved field 302, a 12-bit MPDU length field 304, an 8-bit Cyclic Redundancy Check (CRC) field 306 for checking a transmission error of the MPDU field 310, which may be generated in a transmission process, and an 8-bit unique pattern field 308.
As described above, the aggregation method of the MAC layer is a method of transmitting a single PSDU by joining and sequentially arranging a plurality of MPDUs, thereby saving resources assigned to a PLCP header and a preamble in transmission of every MPDU. In addition, a time interval, i.e., SIFS, can be saved by sequentially transmitting the plurality of MPDUs. Moreover, even if an error is generated in a certain MPDU during a transmission process, the other MPDUs can be successfully transmitted using the MPDU delimiter field 300.
However, when the number of MPDUs joined into a single PSDU increases, the number of the 4-byte MPDU delimiter fields 300 defined corresponding to the MPDUs also increases. Also, an at least 4 to 7-byte overhead including the padding field 320 corresponding to each of the MPDUs increases according to the number of MPDUs.
The aggregation method has a further problem in that re-transmission for an MPDU in which an error is generated during a transmission process is not performed. Hence, the conventional aggregation method needs improvement as to efficiency.
Thus, a technique for efficiently transmitting a plurality of MPDUs, which minimizes a waste of assigned wireless media and minimizes a transmission delay, is required for a WLAN system.